1. Field of the Invention
This present invention relates to an improved finisher composition for protecting the surface of lithographic printing plates. More particularly, the present invention relates to aqueous finishers suitable for preserving non-image hydrophilicity and image oleophilicity. More specifically, the invention provides a finisher composition that resists damage due to scratching, has fast roll up without blinding, even when stored at high temperatures and humidities, and permits effective corrections to the plate such as additions and deletions.
2. Description or Related Art
The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water and repels the ink while the image area repels the water and accepts the ink. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cardboard and the like. Commonly the ink is transferred to an intermediate material commonly called a blanket which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
The most common type of lithographic plate to which the present invention is directed has a light sensitive coating applied to an aluminum base support. The coating may respond to light by having the portion which is exposed become soluble so that it is removed in the developing process. Such a plate is referred to as positive acting. Conversely, when the portion of the coating which is exposed becomes hardened, the plate is referred to as negative acting. In both instances the image area remaining is ink receptive or oleophilic and the non-image area is water receptive or hydrophilic. The differentiation between image and non-image areas is made in the exposure process where a film is applied to the plate with a vacuum to insure good contact. The plate is then exposed to a light source, a portion of which is composed of UV radiation. In the instance where a positive plate is used, the area on the film that corresponds to the image on the plate is opaque so that no light will strike the plate whereas the area on the film that corresponds to the non-image area is clear and permits the transmission of light to the coating which then becomes more soluble and is removed through development. In the case of the negative plate the converse is true. The area on the film corresponding to the image area is clear while the non-image area is opaque. The coating under the clear area is hardened by the action of light while the area not struck by light is removed by development. The light hardened surface of a negative plate is therefore oleophilic and will accept ink while the non-image area which has had the coating removed through the action of the developer is desensitized and therefore hydrophilic.
The developed plate, whether negative or positive, is rinsed and treated with a finisher to preserve the plate's image/non-image differentiation until such time that the plate is placed on a printing press to produce copies. It is necessary for the image to remain oleophilic and for the non-image area to remain hydrophilic. The interval between the time a plate is prepared and is run on press may vary from several hours to several weeks. When a plate is being stored, the conditions of storage may range from hot to cold and from dry to humid. It is not uncommon for plates stored under such conditions to manifest a variety of maladies which render the plate difficult to impossible to use. Performance deficiencies are characterized as slow roll up or blinding in the image area and toning or scumming in the background. This is particularly true for plates stored under high temperature and/or high humidity conditions. Additionally, plates may be stored under lighted conditions which causes light hardening of the finisher components thereby resulting in slow roll up or, in severe cases, blinding of the image. With the advent of aqueous plate systems, problems associated with finishers have become more acute. This is particularly true of slow roll up and blinding. In order to provide plates that are developable with less aggressive developers, coatings had to be modified. These modifications generally increase the susceptibility to slow roll up and blinding. To a lesser degree, but still important, is the background problem of scumming. Background sensitivity to ink has occurred primarily through the dilution of finisher compositions to minimize slow roll up.
An additional obstacle to the successful use of a finisher composition is the scratching problem associated with finished plates. Even when great care is taken, the handling of processed plates can cause scuffing and/or scratching of the surface. When this occurs in the image area it is usually not a problem unless the scratch is severe. However, when the background is damaged the areas affected become ink receptive. In this event the plate requires correction by a chemical treatment or by honing. Often the plate must be discarded.
To minimize either blinding or scumming, it is conventional to treat a freshly developed plate after water rinsing with a plate finisher that normally contains a hydrophilic colloid, a surfactant, salts and water.
Gum arabic and synthetic gums have been used to finish lithographic printing plates. However, with these agents, gum blinding often occurs.
Improved finishers have been prepared and used which contain water, tapioca dextrin, an anionic surfactant, and as a humectant, glycerin. Sporadic blinding occurred despite the improvement; performance was not consistent. The tapioca dextrin required prolonged heating to dissolve it. Another improved finisher is described in U.S. Pat. No. 4,162,920, which finisher is additionally a preserver. This finisher uses tapioca dextrin as the hydrophilic colloid. Other ingredients are a mixture of anionic and nonionic surfactants, glycerin and a petroleum distillate to dissolve the nonaqueous surfactant. An emulsion is formed. Despite freedom from blinding, as it is an emulsion, the finisher settles upon standing and is not usable for machine processing.
U.S. Pat. No. 4,400,481 teaches the use of a natural gum such as gum arabic and a synthetic gum such as polyacrylamide. This composition provides a clean background upon printing but has increased viscosity due to the use of homopolymeric polyacrylamide and exhibits slow roll up on aqueous plates. Additionally, the background is easily damaged which causes ink to adhere to the affected areas.
U.S. Pat. No. 4,213,887 teaches the use of dextrin or polyvinyl pyrrolidone as the hydrophilic polymer in combination with a nonionic surfactant, a humectant and an inorganic salt. This composition provides moderate roll up but does not improve over the inherent hydrophilicity of the background nor provide quick roll up with aqueous plates.
U.S. Pat. No. 4,033,919 teaches the use of copolymers of acrylamide and carboxyl containing monomers as substitutes for gum arabic in desensitizing lithographic plates The compositions as described are effective in rendering a hydrophilic background but seriously affect the ink receptivity of the image area.
U.S. Pat. Nos. 4,246,843 and 4,266,481 teach the use of carboxylated polymers of polyacrylamide. These compositions are effective in providing a hydrophilic surface but offer no resistance to scratching or storage at high temperatures and/or humidities. Additionally, the image is prone to have slow roll up or even blinding.